Hydridorhodium catalyst

The rates of activated alkene hydrogenations seen in the a-bove studies are comparable to those seen earlier by Wilkinson when RhCl(PPh3) was reportedly formed in situ from PPI13 and [RhCl(C0D)]2 (21) supporting our conclusion that removal of phosphine was responsible for the activations seen in our procedures. However alternative explanations involving possible formation of a hydridorhodium catalyst (e.g. eq 4 and 5 must also be considered. The hydridorhodium catalysts formed in these equa-... 

Several observations suggest that HC1 absorption to form a hydridorhodium catalyst is not the mode of activation seen in the 1-hexene, cyclohexene, and thylene hydrogenations discussed a-bove. First, PS-S03 H2NMe2, which is a good HC1 absorber, does not activate any of these alkene hydrogenations with or without ethylene pretreatment. Second, addition of neutral alumina (another HC1 absorber) or powdered KOH at KOH/Rh molar ratios of 20/1 was ineffective at producing a rate acceleration under our conditions. Larger ratios of KOH/Rh (e.g. 100/1) did accelerate... 

Chiral diphosphites based on (2R,3R)-butane-2,3-diol, (2R,4R)-pentane-2,4-diol, (25, 5S)-hexane-2,5-diol, (lS -diphenylpropane-hS-diol, and tV-benzyltartarimide as chiral bridges have been used in the Rh-catalyzed asymmetric hydroformylation of styrene. Enantioselectivities up to 76%, at 50% conversion, have been obtained with stable hydridorhodium diphosphite catalysts. The solution structures of [RhH(L)(CO)2] complexes have been studied NMR and IR spectroscopic data revealed fluxional behavior. Depending on the structure of the bridge, the diphosphite adopts equatorial-equatorial or equatorial-axial coordination to the rhodium. The structure and the stability of the catalysts play a role in the asymmetric induction.218... 

Alternatively, the rhodium dimer 30 may be cleaved by an amine nucleophile to give 34. Since amine-rhodium complexes are known to be stable, this interaction may sequester the catalyst from the productive catalytic cycle. Amine-rhodium complexes are also known to undergo a-oxidation to give hydridorhodium imine complexes 35, which may also be a source of catalyst poisoning. However, in the presence of protic and halide additives, the amine-rhodium complex 34 could react to give the dihalorhodate complex 36. This could occur by associative nucleophilic displacement of the amine by a halide anion. Dihalorhodate 36 could then reform the dimeric complex 30 by reaction with another rhodium monomer, or go on to react directly with another substrate molecule with loss of one of the halide ligands. It is important to note that the dihalorhodate 36 may become a new resting state for the catalyst under these conditions, in addition to or in place of the dimeric complex. 

By contrast the silyl complexes are important catalysts in a variety of hydrosilylation reactions.20 They can be prepared by the similar oxidative addition of hydrosilanes to rhodium(I) complexes, either directly or in solution (equation 189).935 However, in the presence of both additional base and triphenylphosphine a hydridorhodium(I) complex results (equation 190).936 Alternatively in the presence of a large excess of hydrosilane the monohydrido complex is transformed into a dihydrido complex.922... 

Carbon monoxide reduces aromatic nitro compounds when iron pentacarbonyl is used as catalyst." A direct homogeneous catalytic reduction of nitro derivatives with water under moderate carbon monoxide pressure also occurs when rhodium carbonyl derivatives in aqueous organic bases are used as catalysts (equation 21). Presumably hydridorhodium carbonyl species are the active agents whose preferred formation in aqueous organic base may be analogous to that of iron carbonyl hydrides. 

The dinuclear hydridorhodium complex [ ( x-H)Rh P(0— Pr )3 2 2] is a catalyst for the stereoselective hydrogenation of dialkylalkynes and diarylalkynes to the corresponding rrans-alkenes. Although the hydrogenation rates are much lower than for terminal alkenes (approximately 1 turnover... 

Non-oxidative isomerizations often occur when olefinic compounds react with noble metal compounds, e. g., in Wacker oxidation of higher olefins. An example is found in the oxidation of 1-octene where octane-2-, 3-, and 4-ones are formed, in this example with an immobilized Pd" catalyst [130]. A plausible mechanism with a hydridorhodium species as catalytically active moiety has been described by Cramer [131]. 

These latter transformed the catalyst into a hydridorhodium(I)phosphine complex and enabled the catalytic reduction of different dialkyl-, alkyl-aryl-, diaryl-, and cyclic ketones with acceptable reaction rates under mild conditions (50 , 1 bar). 

Another set of insertions of olefins into metal hydrides that has been studied in depth are those involving Rh(III) hydrides that are relevant to the hydrogenation of olefins by Wilkinson s catalyst. An insertion of cyclohexene into such a complex is shown in Equation 9.45a. The cyclohexyl hydride complex is not observed directly because reductive elimination to form cyclohexane is fast. However, the related insertion of ethylene into the PPhj-ligated hydridorhodium dichloride forms a stable ethyl complex (Equation 9.45b). ... 

The structures of the hydridorhodium complexes that are present in the catalytic system have been deduced by NMR spectroscopy. Brown showed that HRh(CO)2(PPh3)j exists as an 85 15 mixture of diequatoriakapical-equatorial isomers of HRh(CO)2(PPh3)2 (Scheme 17.11) that undergoes rapid equilibration at room temperature. This rapid equilibration of trigonal bipyramidal complexes could occur by either a Berry pseudorotation mechanism or a turnstile mechanism. In situ IR transmission spectroscopy on the catalytic system demonstrated that these two isomers are the resting state of the catalyst and were by far the predominant species present during hydroformylation of 1-octene (60-100 C, 5-20 atm, [Rh] = 1 mM, PPhj/Rh = 5). °... 

Cl-Rh dimer. Stoichiometric reaction of hydrosilanes with both catalysts indicated the formation of hydridorhodium intermediates and the absence or low-level formation of dehydrogenative silylation products is suggestive of a Chalk-Harrod mechanistic pathway for the catalysis. 

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